In the prior art, white LEDs are made using blue LEDs with a silicone/phosphor mixture on top of an emitting region for the LEDs. The mixture converts blue light to white light. Current technology largely uses inorganic phosphor powders mixed in silicone for the mixture. The silicone is optically transparent, serves as a binder for the phosphor, and also has an index of refraction intermediate between the chip and air for better extraction of blue and white photons.
Phosphor powders are weighed and blended to achieve the correct color targets characterized by CCT and CRI specifications of the LED manufacturer; targets which are tailored for specific applications and market segments (illumination, display, automotive, flash, etc.). Illumination, in particular, requires a large number of phosphor-silicone combinations that cover a wide range of CCTs and CRIB. In particular, high CRI is required when providing proper color rendering in retail, hospitality and some home applications.
Conventional inorganic phosphors are typically stored in powder form. For direct on-LED applications, the process for combining the phosphor powders with LEDs involves mixing phosphor powder in the A and B parts of the silicone, dispensing the resulting slurry in an LED package, followed by temperature curing which enables crosslinking of the A and B silicone materials. Other diffuser materials are typically added to the phosphor/silicone mix, for example, to improve color over angle at the LED level.
Phosphor blends have to be adjusted for the LED-emitting wavelength, which typically has a wide range (5-20 nm) in blue LED production runs. This requires tight binning of the LEDs before and after the application of the phosphor/silicone mixture in order to achieve tight CCT and CRI tolerances at the product level. Phosphor powders in A/B silicone have a limited shelf life and need to be dispensed relatively quickly in order to prevent settlement and agglomeration of the powder in silicone as well as to avoid the onset of cross-linking. Automating the color targeting and dispensing equipment for traditional phosphors is difficult due to the abrasive nature of the phosphor materials (requires hardened steel tooling), the short life of silicone mixes, and self-absorption in the various powders. The prior art solution of binning LEDs rather than automatically adjusting the dispensed mixture is very expensive and complex.
Quantum dots (QDs) having a high photoluminescence quantum yield (PLQY) may be applicable as down-converting materials in down-converting nano-composites used in solid state lighting applications. Down-converting materials are used to improve the performance, efficiency and color choice in lighting applications, particularly light emitting diodes (LEDs). In such applications, quantum dots absorb light of a particular first (available or selected) wavelength, usually blue, and then emit light at a second wavelength, usually red or green.
QDs may contain Cadmium (Cd), the use of which is regulated in many countries, especially when used in powder form. As a safer form of delivery, QDs are typically stored in non-polar solvents like toluene and cyclohexane and can be eventually delivered from solvent into silicone mixtures that are directly applied to the emitting region of the blue LED or in the form of a remote phosphor.
Transporting QDs in solvent or uncured silicone mitigates the hazards of nano-powders; however, solvents like toluene are toxic and/or flammable and require special handling. Furthermore, transporting the QDs in silicone may limit the final silicone choice for the LED and QD composite.
Silicone paste is commonly used when applying color to injection-molded silicone parts. Silicone paste is highly viscous, typically formed from a non-crosslinking form of silicone resin (e.g., polydimethyl siloxane-based fluid), optional solvent, and optional additives, all of which are compatible with the storage and delivery of QDs. Silicone paste will not crosslink with the A and B silicones used in molding parts, so the paste can be used with any number of A/B silicones from multiple vendors. Furthermore, silicone paste has a long shelf life, and color pigment in the paste tends to stay in suspension with minimum settling or agglomeration. Injection molding machines typically have streams for the A and B silicones. Color is introduced as a paste through a third stream and mixed into the A and B silicones before the parts are formed and cured.